Sandspit (landform)

A spit or sandspit is a deposition bar or beach landform off coasts or lake shores. It develops in places where re-entrance occurs, such as at a cove's headlands, by the process of longshore drift by longshore currents. The drift occurs due to waves meeting the beach at an oblique angle, moving sediment down the beach in a zigzag pattern. This is complemented by longshore currents, which further transport sediment through the water alongside the beach. These currents are caused by the same waves that cause the drift.

Where the direction of the shore inland re-enters, or changes direction, for example at a headland, the longshore current spreads out or dissipates. No longer able to carry the full load, much of the sediment is dropped. This is called deposition. This submerged bar of sediment allows longshore drift or littoral drift to continue to transport sediment in the direction the waves are breaking, forming an above-water spit. Without the complementary process of littoral drift, the bar would not build above the surface of the waves becoming a spit and would instead be leveled off underwater.

Spits occur when longshore drift reaches a section of headland where the turn is greater than 30 degrees. The spit will continue out into the sea until water pressure (e.g. from a river) becomes too great to allow the sand to deposit. Vegetation may then start to grow on the spit, and the spit may become stable and often fertile. A spit may be considered a special form of a shoal. As spits grow, the water behind them is sheltered from wind and waves, and a salt marsh is likely to develop.

Wave refraction can occur at the end of a spit, carrying sediment around the end to form a hook or recurved spit. Refraction in multiple directions may create a complex spit. Waves that arrive in a direction other than obliquely along the spit will halt the growth of the spit, shorten it, or eventually destroy it entirely.

The sediments that make up spits come from a variety of sources including rivers and eroding bluffs, and changes there can have a major effect on spits and other coastal landforms. Activities such as logging and farming upstream can increase the sediment load of rivers, which may hurt the intertidal environments around spits by smothering delicate habitats. Roads or bulkheads built along bluffs can drastically reduce the volume of sediment eroded, so that not enough material is being pushed along to maintain the spit.

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